Some network operators are building out cellular networks, such as small cellular base stations, to handle the growth in mobile data consumption. The data traffic is sent to a back haul network to a core of the mobile network. Different options for the back haul network may be available, such as fiber and microwave; however, hybrid fiber co-axial (HFC) networks may also be used.
Mobile base stations need to be frequency and phase synchronized to guarantee handover performance for subscriber end devices. For example, when a mobile device switches over from one mobile base station to another mobile base station, these base stations need to be synchronized to ensure the correct handover from mobile base station to mobile base station for a subscriber end device. The back haul network typically provides the frequency and timing (e.g., phase) synchronization to the mobile base stations.
Mobile networks may use a timing protocol, such as Institute of Electrical and Electronics Engineers (IEEE)-1588, to provide synchronization over the network. IEEE-1588 in addition to other timing protocols assumes that the network delay in the upstream and the downstream paths is symmetrical. Any delay asymmetry in the network will effectively cause a phase error of 0.5 of the asymmetry value. In addition, the timing synchronization is sensitive to packet delay variations (PDV) in the network. A large PDV will cause timing synchronization errors.
An HFC network may use a protocol, such as data-over-cable service interface specification (DOCSIS), to deliver packets over the back haul network for the mobile network. As DOCSIS is a packet-based network, some network asymmetry in packet delay variation may be introduced in the DOCSIS network. For example, DOCSIS presents challenges due to asymmetry of the nature of the DOCSIS upstream scheduling, jitter (PDV) due to the upstream scheduling, low-bandwidth channels that cause larger packet transition time, and unknown delays in asymmetries in a cable modem termination system (CMTS), cable modem physical devices, and the HFC network. Attempting to send packets with timing information on top of a DOCSIS network does not provide the required timing and frequency accuracy needed for a mobile network.
DOCSIS may use a timing protocol, such as DOCSIS timing protocol (DTP), that synchronizes the frequency and time of the network. Frequency is addressed by coupling the cable modem Ethernet timing to the DOCSIS downstream baud clock. The time synchronization is addressed by coupling a cable modem timestamp message to a DOCSIS extended timestamp. The time offset in asymmetry is addressed through measurement signaling and ranging. To address the asymmetry, the DOCSIS system needs to measure multiple times within the HFC network. This results in a large number of measurements to be made. The large number of measurements then complicates calculation to perform the synchronization.